Diabetes mellitus is an insidious disease which affects more than 15 million Americans. About 1.5 million of these are Type I diabetics (insulin-dependent) and 12 to 14 million are Type II diabetics (noninsulin-dependent). The characteristics of diabetes include chronic and persistently high levels of glucose in blood and in urine. Although urine glucose has been used to monitor glucose levels, the measurement of blood glucose is more reliable and logistically feasible. Blood glucose has therefore become the most commonly followed clinical marker for monitoring the progress of diabetes (and other diseases) to determine treatment and control protocols. Glucose levels are routinely measured in doctors' offices, clinical laboratories, and hospitals. However, the most convenient and important measuring is in-home self-monitoring of blood glucose levels by the patients themselves to permit adjustment of the quantities of insulin and hypoglycemics administered.
There are many products for diabetes related testing of glucose for diagnostic and monitoring purposes. Most of the currently available technologies, especially for self-monitored blood glucose measurements, are not satisfactory because they require some kind of deep lancing or finger stick with associated pain and sometimes excessive bleeding.
The smallest lancet or needle currently marketed for blood sampling has a diameter between 300 micrometers and 500 micrometers, and is constructed of stainless steel with beveled edges. Due to the large cross-section of these lancets, fingertip lancing is painful and frequent lancing causes calluses, impairment of the use of hands, psychological trauma and other unpleasant consequences. Further, blood samples recovered from the patient must be transferred to a test strip or cartridge for assaying analyte concentrations. Obtaining blood samples by lancing and performing the analysis can be messy as well as painful for the patient.
U.S. Pat. No. 5,801,057, “Microsampling Device and Method of Construction,” issued Sep. 1, 1998, to Wilson H. Smart and Kumar Subramanian, describes a self-contained microsampling device and method for the measurement of glucose and other analytes in blood. Blood is drawn through a microneedle sufficiently small that the sampling is virtually painless into an integrated microcuvette where the analyte concentration is measured. The microsampling device of Smart et al has two windows, namely, a glass film for one window and a glass wafer for the other window. While these windows provide excellent transparency and functionality, the application of semiconductor processing to glass is less well established than is the case for silicon nitride. A silicon nitride cuvette window can be directly integrated with the other components of the microsampling device using standard semiconductor processing.
Unsupported silicon nitride films are used as membranes in products such as condenser microphones and pressure sensors. Supported films are used in electronics applications. In Yoo et al, U.S. Pat. Nos. 5,578,517 and 5,729,041, a silicon nitride film is used to form a transparent window covering a fusible link. The window is fully supported on the fusible link or other substrate materials, however. Further, the window is not used to permit optical measurements but rather to permit laser irradiation of the fusible link.